A 24/7 SMP server with a loose pre-industrial theme, so no giant towers with lava-falls, huge glass domes or underwater cities. Although its not very fleshed-out, we use paper as a currency to buy blocks from each other.

Location: Eastern Canada (GMT-4)

Running hey0's admin mod with llamacraft anti-griefing mod.

RULES:
No buildings unfeasible in pre-industrial times.
No stealing
No reed farms (growing money)
Don't use obsidian for anything
No griefing (duh)

This is the single best server ever. I love that it has a bit of an economy, but it is sort of ineffective. There is always a surplus of paper. I do think the currency should be changed to iron, but that can be done by each citizen.

I think gold is the best currency. Its relatively rare, and as of now has no intrinsic value. Maybe gold coins would be a useful addition to Minecraft. A system that allows admins to spawn paper money could also be useful if the admins know what they are doing.
Having a currency that is easy to make, like normal paper, isn't going to work.